Résumé :
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1H-NMR imaging combined with arterial spin labeling (ASL) provides dynamic, quantitative and non invasive measurements of muscle perfusion as well as blood oxygenation level dependent (BOLD) contrast. In Parallel, 31P-NMRS has long been used to investigate muscle exercise metabolism but remains challenging in mouse because of the small size of the animal. The cross analysis of these three parameters is rarely explored, however valuable for an accurate characterization of muscular diseases. In this work, a complete experimental set-up has been developed to simultaneously and non-invasively assess perfusion, BOLD and phosphorus metabolites in mouse, in response to electrical stimulation. Experiments were performed inside a Bruker Biospec 4T NMR system using custom-built coils. Muscle contractions were induced with an electrostimulator through subcutaneous electrodes. In order to compensate for minimal 31P signal, we explored the possibility to sum the results of 10 consecutive exercises. C57/bl6 mice were repeatedly subjected to a "rest(30s)- exercise(30s)- recovery(9min)" protocol. Perfusion/BOLD and phosphocreatine content (PCr) were monitored with a time resolution of 9s and 2s, respectively. Time courses of rephosphorylation of PCr were fitted with a single exponential function. Exercise systematically resulted in a total consumption of PCr. Owing to high time resolution, we could measure the subsequent initial rate of PCr recovery, a direct estimate of mitochondrial oxidative capacity, to have a time constant ?PCr= 77 +/- 16s. Perfusion was 6.6 +/- 3.4 ml.min-1.100g-1 at rest and rose to 141 +/- 13 ml.min-1.100g-1 immediately after the end of the exercise. We also observed a transient increase in BOLD signal. This is the first report of a totally non-invasive assessment of mouse skeletal muscle function in vivo by combined NMR imaging and spectroscopy. This tool offers new perspectives to study muscle pathology in mice models and monitor longitudinal effect of therapeutic protocols.
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